1. Technical Field
The present invention relates to Fourier transform lens systems and a holographic storage system using a Fourier transform lens system.
2. Related Art
Among all of modern systems for data storage, a holographic storage system is believed by many to be the most promising due to its higher data storage density and parallel read/write operation. One kind of holographic storage system is a volume holographic storage system.
In a holographic storage system, data is stored as holograms that result from the interference of a signal beam and a reference beam. Generally speaking, during a data recording process, a laser light beam is split into two coherent beams. One of the coherent beams is referred to as the signal beam, and the other coherent beam is referred to as the reference beam. Data can be encoded within the signal beam by using an encoding device, e.g., a spatial light modulator (SLM). Typically, an SLM is a two-dimensional matrix of pixels. Each pixel in the matrix can be directed as follows: to transmit or reflect light, which corresponds to the binary digit 1; or to block light, which corresponds to the binary digit 0. Once the signal beam is encoded by the SLM, it passes through a Fourier transform lens system, and is incident on a holographic storage medium where it intersects with the reference beam to form an interference pattern (i.e., hologram). The interference pattern records the data encoded in the signal beam to the holographic storage medium. During a data retrieval process, the data recorded in the holographic storage medium is read by illuminating the storage medium with the reference beam. The reference beam diffracts off the stored hologram, generating a reconstructed signal beam proportional to the original signal beam used to store the hologram. The reconstructed signal beam passes through a Fourier transform lens system, and is then typically imaged onto a sensor such as a CCD (charge coupled device) or a CMOS (complementary metal oxide semiconductor) active pixel array device. The sensor is attached to a decoder, which is capable of decoding the data contained in the reconstructed signal beam.
Generally, the above-described data recording process and retrieval process can be accomplished by a single integrated holographic storage system or by two separate holographic systems. In the case of two separate holographic systems, one of these is a holographic recording system for data recording, and the other is a holographic retrieval system for data retrieval. The implementation of holographic recording and retrieval techniques in a commercially viable storage system benefits from a simple and robust design of the Fourier transform lens systems.
Conventionally, in order to obtain a holographic storage system having a very high storage density, an asymmetrical Fourier transform lens system which has a relatively large field of view can be employed. However, the conventional asymmetrical Fourier transform lens system is usually configured to have more than five spherical lens pieces. This makes the holographic storage system unduly awkward and bulky.
What is needed is a Fourier transform lens system having a compact configuration, and a holographic storage system having such kind of Fourier transform lens system.